Scientists and animal rights activists have long wondered if wheel running was a neurotic behavior exclusive to captive mice. But new research on mice in the wild shows that it's possible these rodents run for fun.

Johanna H. Meijer and Yuri Robbers, researchers at the Leiden University Medical Center in the Netherlands, placed exercise wheels outdoors in two areas—one urban, and one not accessible to the public—in partially enclosed cages to keep larger animals from knocking them over. They pointed video cameras triggered by infrared motion detectors on them. Over the course of the three years, cameras shot 200,000 clips of the wheels in motion.

After analyzing 12,000 clips, the researchers found that feral mice did, in fact, use the wheels: The cameras recorded 734 cases of mice running on the wheel in the urban area, and 232 cases in the private area, over the course of 24 months. Wheel running occurred year-round; the mice were typically juvenile. The mice spent between one and 18 minutes on the wheel. The animals got on the wheel, got off, and within minutes got back on, running away—and they always ran. They never walked.

“When I saw the first mice, I was extremely happy,” Meijer told the New York Times. “I had to laugh about the results, but at the same time, I take it very seriously. It’s funny, and it’s important at the same time.”

Though mice accounted for the vast majority of wheel running observations (88 percent), they weren't the only animal to go for a spin on the wheels: shrews, rats, slugs, frogs, and snails also hopped on (the snails, however, were excluded from the analysis, because they "caused haphazard rather than directional movement of the wheel," according to the study, which appeared in Proceedings of the Royal Society B). Like mice, some shrews, rats, and frogs left the wheel and then entered it again to keep running. "This observation indicates that wheel running may well be intentional rather than unintentional for these animals," the study says.

Though the enclosures at first had food, removing the food didn't stop mice from running. Although the researchers noted that the number of visits to the enclosures dropped after the food was removed, "the fraction of visits including wheel running increased. This implies that wheel running can be experienced as rewarding even without an associated food reward, suggesting the importance of motivational systems unrelated to foraging."